Long Term Evolution (LTE) is the name of the 3rd Generation Partnership Project (3GPP) evolution of the Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access and Radio Access Network (UTRAN), published in 3GPP Release 8 and subsequent specification.
LTE is based upon OFDMA (Orthogonal Frequency Division Multiple Access) in DL (downlink) and SC-FDMA (Single Carrier Frequency Division Multiple Access) in UL (uplink) using modulations up to 64-QAM (Quadrature Amplitude Modulation). The sub carrier separation is 15 KHz and the possible bandwidth allocations range from 1.4 MHz to 20 MHz, and even more in case of aggregation.
A requirement on the LTE standard is that it should be simpler to deploy and manage than previous 3GPP standards such as GSM (Global System for Mobile Communications) and WCDMA (Wide Code Division Multiple Access). LTE should also be compatible with these systems meaning, in other words, that there is full support for handover between the new and old standards. Spectrum usage is also standardized. By creating new access technology and rolling out new systems, more interference is added to already existing interference which may cause performance degradation to some or all systems.
A topic of ongoing discussions in the industry is how to also combine and reuse as much hardware as possible between the systems. Hardware reuse can be considered at three levels:
1. A common cabinet;
2. A common radio (multi-standard radio, or MSR); and
3. A common digital part.
In the multi-standard products, different standards are expected to share cabinet and radio. They may even share digital parts at some point, but not necessarily initially. This implies that a common radio would be capable of performing different RATs (radio access technology) radio functionality at the same time. In a mixed RAT, different standards functionality is based on frequency multiplexing. In a Mixed RAT mode defined by 3GPP, the radio handles at least two RATs both in TX (transmit) and RX (receive) chains simultaneously. Radio chains include different components performing functions such as filtering, signal amplification, up/down RF (radio frequency) conversion to/from base band frequency and gain control among others.
In a geographical area in which different uncoordinated RATs operate, in addition to large number of base stations and mobile stations, there will be a lot of different signals in the air. Some of these signals are desired for a specific base station, functional in a specific RAT. But at the same time, these same signals represent interference for other base stations and RATs. FIG. 1 depicts a representative area in which numerous base stations—which may operate in different RATs—communicate with numerous mobile stations. The solid lines represent the desired signals, while the dashed and dotted lines are considered as interferences.
The interfering signals that come into the RX chain cause performance degradation. To mitigate performance degradation, reducing interference in the receiver is necessary, which can be accomplished with different methods. These methods in general can be categorized as filtering and dynamic mitigation. In the filtering method, interference is attenuated. This method is based on predetermined interference level to which the interference needs to be attenuated so that performance degradation is prevented or minimized. Filtering to attenuate the interference can be implemented in several stages, e.g., in duplex and channel filters.
In dynamic mitigation, the RX gain is a function of a level of interference and is changed in different levels via AGC (Automatic Gain Control) functionality.
There are advantages and disadvantages in both mitigation methods regarding cost and performance. Normally, the most efficient way to mitigate interference is the filtering method that attenuates the interference level down to the predetermined low power levels. This low power level is determined in the design phase and all filtering would be based on this level. Generally, RBSs (radio base station, also referred to as Node B, eNodeB) are designed using both filtering and dynamic methods to handle interferences.
Interference causes performance degradation by adding more noise to the system, which results in a lower throughput. Remedying performance degradation due to interference means stopping or reducing the incoming power level of the interfering signals to the system. Conventionally, this is done by one of the ways mentioned above: filtering or dynamic mitigation. The filtering method means that a chain of filtering in RX is needed to attenuate the interference to a predetermined low power level. The dynamic method is based on changing the RX gain, which depends on component characteristics such as the IIP3 (third-order input intercept point), gain and so on. Both methods can be combined, but the filtering method is preferred since the dynamic method typically adds more noise to the system.
Filtering is performed in at least two stages: duplex filtering (also referred to as band pass filtering) and channel filtering in both analog and digital domains. A duplex filter is a high performing filter that extracts a specific operating band while a channel filter filters wanted signals. The duplex filter is usually used to reduce out-of-band interferences. This means that the duplex filter cannot filter in-band interferences, which is a situation that can be exacerbated in case of an MSR due to several standards.
The filtering method is based, as mentioned above, on reducing assumed interference level to a predetermined low power level. The problem with conventional filtering is that it requires high performance duplex filters to reduce the out-of-band interference, which often are over dimensioned and expensive in practice.
There are several deficiencies associated with conventional filtering. First, the filters are dimensioned to cope with a worst case scenario in terms of near/adjacent interference rejection. But in reality, many sites do not experience such levels of near/adjacent interferences. As a result, the filtering capacity is excessive for many sites resulting in expenditure of unnecessary costs.
Second, the character of the interferences can change over time, and it is all but impossible to anticipate all types of interferences that a site will experience. As a result, the existing filter may not offer any protection from new interferences that arise.